1. Field of the Invention
The present invention relates to a tag used in an RFID system, namely, a wireless IC tag, and more particularly, to a tag antenna used for such a wireless IC tag, and a tag mounting the tag antenna and an IC chip.
2. Description of the Related Art
RFID (Radio Frequency IDentification) systems are widely used for the management, etc. of objects, or the like. In these systems, a reader/writer emits a radio wave to a tag, the tag returns to the reader/writer information within the tag by a radio wave, and the reader/writer reads the information within the tag. The band of the radio wave is a UHF band. Frequencies in the vicinities of 868 MHz, 915 MHz, and 953 MHz are used in Europe, the United States, and Japan respectively. Depending on the performance of a chip mounted within the tag, a communication distance is approximately 3 to 5 m, and the output of the reader/writer is on the order of 1 W.
There is an advantage in using a dipole antenna as an antenna of such a wireless IC tag that a favorable directivity can be obtained. However, the efficiency of the antenna is maximized when the length of the antenna is one half of the wavelength λ of the radio wave. This leads to a problem that the length of the antenna increases, which in turn disables the downsizing of the tag. Additionally, if there is a metal in the neighborhood of such a dipole antenna being used, the communication distance of the tag significantly decreases.
For example, a patch antenna is conventionally used as an antenna used for a tag attached to a metal. FIG. 1 explains a conventional example of such a patch antenna. In this figure, the patch antenna is composed of a patch conductor 100, a ground conductor 101 on the back surface of a dielectric 102, and the dielectric 102 interposed between the patch conductor 100 and the ground conductor 101. An IC chip is mounted in a chip mounting part 103 on the side of the patch conductor 100. One of terminal electrodes of the IC chip is connected to a suitable portion of the patch conductor 100 positioned on the front surface, whereas the other of the terminal electrodes is connected to the back surface, namely, the ground conductor 101 via a through hole 104.
FIG. 2 shows an example of the communication distance of the patch antenna shown in FIG. 1. For example, if the size of the IC chip is implemented as a 1-mm-square, and the number of tags n is 1, 3 m is obtained as the communication distance at the frequency of 953 MHz. However, for example, if a plurality of identical tags are used in a close range, namely, if the number of tags n is 2 or 3, the characteristic curve of the communication distance shifts to the side of low frequencies, and the communication distance at the frequency of 953 MHz significantly decreases.
Patent Documents 1 to 4 disclose the conventional techniques related to such a wireless IC tag, and an antenna used for such a tag. Patent Document 1 discloses the non-contact IC tag that can hold the read/write state of data constant regardless of a substance positioned on the back surface of the tag by comprising an antenna and a reflection plate with a spacer interposed in between in a structure similar to that shown in FIG. 1.
Patent Document 2 discloses a planar antenna that can reduce an impedance by providing a notch in a folded structure, and can match the impedance to that of the feeding line of 50Ω without requiring an impedance converting circuit, etc.
Patent Document 3 discloses the technique for providing a patch antenna, which has ground and antenna surfaces sandwiching a dielectric in a similar manner, and in which a hole for causing the dielectric to protrude from the antenna surface is provided, and a region sectioned by the protruding dielectric from the hole on the antenna surface forms a matching circuit for a transmission/reception element.
Patent Document 4 discloses the technique for implementing a wireless IC tag with high directivity by using a microstrip antenna where a hook-shaped slit is formed in a mounting portion of a chip on an emission conductor located on the front surface of a dielectric.
However, for example, according to Patent Document 1, the distance between the antenna surface and the reflection plate is equal to or longer than 30 mm when the read distance is maximized, and the thickness of the spacer increases, leading to difficulties in downsizing the IC tag. Also the conventional example shown in FIG. 1 and the techniques disclosed by Patent Documents 2 to 4 cannot solve the problems that a cost is increased by making a through hole, and a communication distance decrease when a plurality of tags are used in a close range, and have difficulties in downsizing an antenna while holding a practical communication distance.
[Patent Document 1] Japanese Published Unexamined Application No. 2002-298106 “Non-contact IC Tag”
[Patent Document 2] Japanese Published Unexamined Application No. 2006-140735 “Planar Antenna”
[Patent Document 3] Japanese Published Unexamined Application No. 2006-237674 “Patch Antenna and RFID Inlet”
[Patent Document 4] Japanese Published Unexamined Application No. 2006-311372 “Wireless IC Tag”